INTRODUCTION
  ------------
  
  Because not every I2C or SMBus adapter implements everything in the 
  I2C specifications, a client can not trust that everything it needs
  is implemented when it is given the option to attach to an adapter:
  the client needs some way to check whether an adapter has the needed
  functionality. 
  
  
  FUNCTIONALITY CONSTANTS
  -----------------------
  
  For the most up-to-date list of functionality constants, please check
  <linux/i2c.h>!
  
    I2C_FUNC_I2C                    Plain i2c-level commands (Pure SMBus
                                    adapters typically can not do these)
    I2C_FUNC_10BIT_ADDR             Handles the 10-bit address extensions

    I2C_FUNC_PROTOCOL_MANGLING      Knows about the I2C_M_IGNORE_NAK,
                                    I2C_M_REV_DIR_ADDR, I2C_M_NOSTART and
                                    I2C_M_NO_RD_ACK flags (which modify the
                                    I2C protocol!)

    I2C_FUNC_SMBUS_QUICK            Handles the SMBus write_quick command
    I2C_FUNC_SMBUS_READ_BYTE        Handles the SMBus read_byte command
    I2C_FUNC_SMBUS_WRITE_BYTE       Handles the SMBus write_byte command
    I2C_FUNC_SMBUS_READ_BYTE_DATA   Handles the SMBus read_byte_data command
    I2C_FUNC_SMBUS_WRITE_BYTE_DATA  Handles the SMBus write_byte_data command
    I2C_FUNC_SMBUS_READ_WORD_DATA   Handles the SMBus read_word_data command
    I2C_FUNC_SMBUS_WRITE_WORD_DATA  Handles the SMBus write_byte_data command
    I2C_FUNC_SMBUS_PROC_CALL        Handles the SMBus process_call command
    I2C_FUNC_SMBUS_READ_BLOCK_DATA  Handles the SMBus read_block_data command
    I2C_FUNC_SMBUS_WRITE_BLOCK_DATA Handles the SMBus write_block_data command
    I2C_FUNC_SMBUS_READ_I2C_BLOCK   Handles the SMBus read_i2c_block_data command
    I2C_FUNC_SMBUS_WRITE_I2C_BLOCK  Handles the SMBus write_i2c_block_data command
  
  A few combinations of the above flags are also defined for your convenience:
  
    I2C_FUNC_SMBUS_BYTE             Handles the SMBus read_byte
                                    and write_byte commands
    I2C_FUNC_SMBUS_BYTE_DATA        Handles the SMBus read_byte_data
                                    and write_byte_data commands
    I2C_FUNC_SMBUS_WORD_DATA        Handles the SMBus read_word_data
                                    and write_word_data commands
    I2C_FUNC_SMBUS_BLOCK_DATA       Handles the SMBus read_block_data
                                    and write_block_data commands
    I2C_FUNC_SMBUS_I2C_BLOCK        Handles the SMBus read_i2c_block_data
                                    and write_i2c_block_data commands
    I2C_FUNC_SMBUS_EMUL             Handles all SMBus commands than can be
                                    emulated by a real I2C adapter (using
                                    the transparent emulation layer)

  ADAPTER IMPLEMENTATION
  ----------------------

  When you write a new adapter driver, you will have to implement a
  function callback `functionality'. Typical implementations are given
  below.

  A typical SMBus-only adapter would list all the SMBus transactions it
  supports. This example comes from the i2c-piix4 driver:
  
    static u32 piix4_func(struct i2c_adapter *adapter)
    {
  	return I2C_FUNC_SMBUS_QUICK | I2C_FUNC_SMBUS_BYTE |
  	       I2C_FUNC_SMBUS_BYTE_DATA | I2C_FUNC_SMBUS_WORD_DATA |
  	       I2C_FUNC_SMBUS_BLOCK_DATA;

    }

  A typical full-I2C adapter would use the following (from the i2c-pxa
  driver):

    static u32 i2c_pxa_functionality(struct i2c_adapter *adap)

    {

  	return I2C_FUNC_I2C | I2C_FUNC_SMBUS_EMUL;

    }

  I2C_FUNC_SMBUS_EMUL includes all the SMBus transactions (with the
  addition of I2C block transactions) which i2c-core can emulate using
  I2C_FUNC_I2C without any help from the adapter driver. The idea is
  to let the client drivers check for the support of SMBus functions
  without having to care whether the said functions are implemented in
  hardware by the adapter, or emulated in software by i2c-core on top
  of an I2C adapter.

  
  
  CLIENT CHECKING
  ---------------
  
  Before a client tries to attach to an adapter, or even do tests to check
  whether one of the devices it supports is present on an adapter, it should

  check whether the needed functionality is present. The typical way to do
  this is (from the lm75 driver):

    static int lm75_detect(...)

    {

  	(...)
  	if (!i2c_check_functionality(adapter, I2C_FUNC_SMBUS_BYTE_DATA |
  				     I2C_FUNC_SMBUS_WORD_DATA))
  		goto exit;
  	(...)

    }

  Here, the lm75 driver checks if the adapter can do both SMBus byte data
  and SMBus word data transactions. If not, then the driver won't work on
  this adapter and there's no point in going on. If the check above is
  successful, then the driver knows that it can call the following
  functions: i2c_smbus_read_byte_data(), i2c_smbus_write_byte_data(),
  i2c_smbus_read_word_data() and i2c_smbus_write_word_data(). As a rule of
  thumb, the functionality constants you test for with
  i2c_check_functionality() should match exactly the i2c_smbus_* functions
  which you driver is calling.
  
  Note that the check above doesn't tell whether the functionalities are
  implemented in hardware by the underlying adapter or emulated in
  software by i2c-core. Client drivers don't have to care about this, as
  i2c-core will transparently implement SMBus transactions on top of I2C
  adapters.

  
  
  CHECKING THROUGH /DEV
  ---------------------
  
  If you try to access an adapter from a userspace program, you will have
  to use the /dev interface. You will still have to check whether the
  functionality you need is supported, of course. This is done using

  the I2C_FUNCS ioctl. An example, adapted from the i2cdetect program, is
  below:

  
    int file;

    if (file = open("/dev/i2c-0", O_RDWR) < 0) {

  	/* Some kind of error handling */
  	exit(1);
    }

    if (ioctl(file, I2C_FUNCS, &funcs) < 0) {

  	/* Some kind of error handling */
  	exit(1);
    }

    if (!(funcs & I2C_FUNC_SMBUS_QUICK)) {

  	/* Oops, the needed functionality (SMBus write_quick function) is
             not available! */
  	exit(1);
    }
    /* Now it is safe to use the SMBus write_quick command */